Radio Frequency (RF) multi-throw switches are critical components in today's mobile devices. They are used to connect a common port of the switch to one of two or more selectable output/input ports. The connection path must exhibit very low loss so as not to degrade system efficiency, while at the same time, the two connected ports must have high isolation from the remaining ports. At the antenna of an RF front-end, for example, the high isolation is very important for protecting the sensitive receiver input from high RF transmit power.
Semiconductor Field-Effect Transistors (FETs) on materials such as GaAs, Silicon-On-Insulator (SOI), etc. are widely used for implementing RF switches. However, in order to achieve the high levels of isolation required, multiple devices must be stacked in series in the through and shunt paths. To change the state of these switches, a Direct Current (DC) control voltage must be selectively applied to the gates of the appropriate FETs. To prevent RF energy from being lost into the control circuit, the control voltages are typically applied through a network of resistors. These resistors impede the flow of the RF energy into the control circuit. To achieve the very low insertion losses required, these resistors are typically tens or hundreds of kilohms (kΩ) in value. While the isolated FETs could be switched from conducting to isolated states extremely rapidly, an RF switch implemented in the manner described exhibits relatively long switching times. This is attributable to the high value of the resistors, through which the control voltage is applied, which, along with internal capacitances inherent in the FET switches, lead to very large RC time constants for the device.
Accordingly, resistors and switches are needed for reducing the switching time of RF switches while maintaining high isolation.